The recovery of hot metal from fine raw materials containing iron oxide, e.g. iron ores, is mainly effected by agglomeration (sintering, pelletizing) followed by smelting reduction (e.g. in a blast furnace). Agglomeration can be avoided by pre-reduction of the fine raw materials to reduced ore and by subsequent smelting of this reduced ore to hot metal. For example, the applicant has developed the so-called “Circofer” process, in which iron ore in the solid state is directly reduced together with coal at e.g. 850-950° C. and a pressure of e.g. 4 bar to obtain sponge iron (cf. WO 2005/116275 A, WO 2005/116273 A). The product of the direct reduction usually is separated by magnetic separation into a non-magnetic fraction of by-products in particular of the coal and a magnetic fraction containing iron. The iron-containing magnetic fraction, i.e. the reduced ore, as intermediate product is then processed further in smelting reduction processes, for example by the so-called “Auslron” process of the applicant or in a submerged arc furnace, to obtain hot metal.
Before the (hot) magnetic separation, the reduced mixture of reduced ore on the one hand and coal ash and residual carbon (char) on the other hand, which is withdrawn from the direct reduction as intermediate product, must at least be cooled to below the Curie temperature of iron of 768° C., in order to achieve an efficient separation of the non-magnetic by-products before smelting. So far, only few concepts are available for cooling the reduced mixture. For example, an indirect cooling by water is proposed. In this case, however, a considerable part of the thermal energy of the reduced mixture from the direct reduction is lost. In accordance with U.S. Pat. No. 4,073,642 the intermediate product is indirectly cooled with air in a classical fluidized bed. In this case, too, a considerable part of the thermal energy is lost, since the air used for cooling cannot be recirculated to the “Circofer” process. Many other publications propose no cooling concept at all.
In the direct reduction of iron ore by the “Circofer” process a reducing fluidizing gas with high temperature is supplied to the fluidized bed for direct reduction. When using greatly carburizing atmospheres (atmospheres with a high CO content) in a temperature range between 450° C. and 800° C., there is a risk of high-temperature corrosion of the plant materials by so-called “metal dusting”. The appearance of this type of damage is not uniform and reaches from general attack to pitting attack. The corrosion product substantially consists of graphite and fine metal particles. “Metal dusting” can lead to a quick material failure of the plant materials. To avoid such high-temperature corrosion, expensive high-alloy steels with high contents of nickel, chromium, aluminum and/or silicon must be used for all components which are in contact with the hot fluidizing gas, such as in particular the gas heater.
The disadvantages of the above-described technology hence consist in considerable energy losses due to the cooling of the iron-containing intermediate products and in the high costs for the gas heater materials to be used.